Ah ok, im with you. Well if you are modelling from an image / painting and have properly matched the camera then when you get all the 3d geometry in place matching the image then it should be creatng a perfect 3d representation with scale and distance all accurate. If not, then its because your camera isnt matched to the scene properly. So really you'll just have to make sure you get a really good match on your camera, and then its straight forward from there :)